The subject of the present invention is a process for producing lith emulsions with improved general sensitivity.
The silver halide emulsions used in the photographic industry for producing lith films must have a very steep gradation and a good dot quality, that is, the dots must possess high density and edge sharpness. This is accomplished by use of emulsions for this purpose having a narrow grain-size distribution and a high proportion of chloride. As a rule, such emulsions contain at least 60 mole percent of chloride and less than 40 mole percent of bromide. However, it is a disadvantage that these emulsions possess a relatively small general sensitivity which is not sufficient for many areas of application in reproduction technology. Thus, for the direct imaging of lith emulsions on electronic color scanners or on reproduction cameras and on reproduction enlargement devices, a high general sensitivity is required which cannot be achieved with the conventional, predominantly chloride-containing emulsions produced by the single or double stream method.
From the German patent No. 1,169,290, published Apr. 30, 1964, photographic silver halide emulsions are known that are characterized by an extremely narrow grain-size distribution. According to one particular embodiment, silver halide emulsions can be obtained according to this process in which the grains display a core/shell structure. However, in practice it has been found that these practically monodisperse emulsions can be sensitized chemically to an adequate extent only with difficulty, so that the high general sensitivity necessary for the present purpose cannot be achieved. This state of affairs is referred to for example in the German Offenlegungsschrift No. 2,042,188, published Mar. 18, 1971. It is also disadvantageous that unusually long precipitation times are required for the production of these emulsions and that, in addition, expensive production equipment with special automatic control and dosing devices, and the like, are necessary. Moreover, it is known that silver halide emulsions can also be prepared by the conversion method, which are characterized by a relatively narrow grain-size distribution. For example, such silver halide emulsions of the conversion type are disclosed in U.S. Pat. No. 2,592,250. As is evident from U.S. Pat. No. 3,703,584 there are also difficulties in raising the sensitivity of these emulsions to a sufficient degree.
Therefore, the object of the present invention is to provide a process for producing silver halide emulsions which possess a narrow grain-size distribution and the typical lith properties, such as steep gradation and good dot quality, but at the same time also possess an extraordinarily high general sensitivity compared to the conventional lith emulsions.